Method and computer program product for oct measurement beam adjustment

ABSTRACT

A method determines translational and/or rotational deviations between the measurement coordinate system of a measurement mirror scanner and the processing coordinate system of a processing mirror scanner. A measurement beam reflected at a workpiece returns on a path of an incident measurement beam and is captured by a spatially resolving measurement sensor to ascertain spatially resolving information of the workpiece. The reflected measurement beam, in a sensor image of the measurement sensor, is imaged onto a previously known image position. This is accomplished by ascertaining a focal position deviation of the processing beam by scanning with the processing beam, evaluating a laser power detected at grid points, fixing the processing mirror scanner, capturing spatially resolving height information of the pinhole diaphragm by the measurement sensor, and determining a translational deviation between the processing and measurement coordinate systems based on the deviation.

The invention relates to a method for determining translational and/orrotational deviations between the measurement coordinate system of ameasurement mirror scanner, which is tiltable about two axes anddeflects a measurement beam, which is generated by a coherencetomography apparatus, for example, two-dimensionally, and the processingcoordinate system of a processing mirror scanner, which is tiltableabout two axes and deflects both the measurement beam deflected by themeasurement mirror scanner and a processing beam two-dimensionally ontoa workpiece, wherein the measurement beam reflected at the workpiecereturns on the path of the incident measurement beam and is captured bya spatially resolving measurement sensor in order to ascertain spatiallyresolving information of the workpiece, and wherein, in a zero positionof the measurement mirror scanner, the reflected measurement beam, inthe sensor image of the measurement sensor, is imaged onto a previouslyknown image position.

Such a method has been disclosed by DE 10 2015 012 565 B3, for example.

For welding fillet weld seams, for example, it is necessary to regulatethe relative position between laser focal spot and workpiece. This weldseam position regulation can be effected by means of so-called opticalcoherence tomography (OCT). This method is based on the basic principleof the interference of light waves and makes it possible to captureheight differences along a measurement beam axis in the micrometersrange. For this purpose, a laser beam is generated by a coherencetomography apparatus and is split into a measurement beam and areference beam by means of a beam splitter. The measurement beam ispassed on to a measurement arm and impinges on a surface of a workpieceto be processed. At this surface the measurement beam is at least partlyreflected and guided back to the beam splitter. The reference beam ispassed on to a reference arm and is reflected at the end of thereference arm. The reflected reference beam is likewise guided back tothe beam splitter. The superimposition of the two reflected beams isfinally detected in order, taking account of the length of the referencearm, to ascertain height information about the surface and/or thecurrent penetration depth of a processing beam into a workpiece.

Prior to being coupled into a common processing scanner, by means ofwhich measurement beam and processing beam can be deflected ontodifferent processing positions, both the processing beam and themeasurement beam pass through various optical elements within theprocessing beam optical system and the measurement beam optical system.Typically it is true that in an initial calibration process the elementsof the respective optical systems for measurement beam and processingbeam are set in such a way that in the case of a predefined zeroposition of these optical elements and their assigned ii deflectiondevices within said optical systems there is congruence of theprocessing beam and the measurement beam on the workpiece. However, if adesired target offset of measurement beam position and processingposition is then set after the calibration process during the processingprocess, the deflection devices assigned to the measurement beam opticalsystem and the processing beam optical system, that is to say theprocessing scanner for jointly deflecting processing beam andmeasurement beam and an upstream measurement scanner for deflecting themeasurement beam, have to be correspondingly adjusted, in which casethey leave the original zero position. This has the effect thatundesired deviations from a target beam path arise in the processingprocess, despite preceding calibration processes, after leaving the zeroposition, with the result that there is no longer congruence between theprocessing beam and the measurement beam. These deviations in therelative position between the two scanners may be caused bymanufacturing-dictated inaccuracies, mounting and various influences,such as e.g. temperature fluctuations during processing. Thesedeviations have to be detected and corrected.

In the method known from DE 10 2015 012 565 B3 cited in theintroduction, the measurement beam position on the workpiece uponadoption of the zero positions of the processing and measurementscanners in a preceding calibration method is determined by a long-timeexposure in which the measurement beam position on the workpiece ishighlighted by an optical marker on the workpiece, said optical markerbeing detectable by the measurement sensor, and the image position ofsaid optical marking in the sensor image of the measurement sensor isdetermined and stored. During the calibration process, the measurementbeam position can be adjusted exactly to the image center, for example.Alternatively, an offset between the measurement beam position and theimage center, said offset being ascertained for the zero position, canbe stored and taken into account as a corresponding error value infurther calculation or regulation processes. If the processing beamposition on the workpiece is subsequently captured in terms ofcoordinates, then a deviation from the image center represents anundesired relative offset between the processing beam position and themeasurement beam position, this offset being able to be ascertained bymeans of known image evaluation algorithms and being compensated for byposition regulation of the measurement scanner. As an alternative to thecalibration method, the reflected measurement beam can be deflected backin the direction of the measurement sensor, such that it is possible toidentify the actual measurement beam position in the captured sensorimage and here, too, an offset is compensated for by position regulationof the measurement scanner.

The present invention is based on the object, in the method mentioned inthe introduction, of determining translational and/or rotationaldeviations between the processing coordinate system of the processingscanner and the measurement coordinate system of the measurement mirrorscanner in a different way.

In the method mentioned in the introduction, the object is achievedaccording to the invention with regard to a translational deviation bymeans of the following method steps (a):

-   -   ascertaining the x-y focal position deviation of the processing        beam relative to the pinhole diaphragm center of a pinhole        diaphragm detector arranged on the workpiece support plane by        scanning the pinhole diaphragm with the processing beam        deflected by the processing mirror scanner in an x-y grid and by        evaluating the laser power detected at each of the grid points        in accordance with the method outlined in DE 10 2011 006 553 A1,        the entire content of which is hereby incorporated by reference,        and by fixing the processing mirror scanner in the scan position        which has been corrected on the basis of the ascertained x-y        focal position deviation, in which scan position the focal        position of the processing beam is situated in a predetermined        position, in particular in the pinhole diaphragm center;    -   with the processing mirror scanner fixed in the corrected scan        position, capturing spatially resolving height information of        the pinhole diaphragm by means of the measurement sensor by        scanning the pinhole diaphragm with the measurement beam        deflected by the measurement mirror scanner; and    -   determining a translational deviation between the processing and        measurement coordinate systems on the basis of the        deviation—present in the sensor image of the measurement        sensor—between the previously known image position corresponding        to the focal position of the processing beam and the pinhole        diaphragm center captured from the height information.

According to the invention, the processing beam is aligned exactly withthe pinhole diaphragm center and then the pinhole diaphragm is scannedby means of the measurement beam deflected by the measurement mirrorscanner. The relative offset in the sensor image between the previouslyknown image position and the pinhole diaphragm center captured in termsof height yields the translational deviation of the two scannercoordinate systems, which can then be compensated for by positionregulation of the measurement scanner, for example.

In the method mentioned in the introduction, the object is achievedaccording to the invention with regard to a rotational deviation bymeans of the following method steps (b):

-   -   deflecting the measurement beam in each case by a positive and a        negative fixed magnitude in the workpiece support plane by        tilting the processing mirror scanner about one, first tilt axis        thereof and, with the processing mirror scanner fixed in each        case in these tilted scan positions, capturing a linear height        edge arranged at the workpiece support plane by means of the        measurement sensor in each case by a line scan of the        measurement beam by deflecting the measurement mirror scanner        about one, second tilt axis thereof, and ascertaining an axis of        the processing coordinate system on the basis of the captured        points of intersection of the two line scans with the height        edge;    -   deflecting the measurement beam in each case by a positive and a        negative fixed magnitude in the workpiece support plane by        tilting the measurement mirror scanner about the other, first        tilt axis thereof and, with the measurement mirror scanner fixed        in each case in these tilted scan positions, capturing the        height edge by means of the measurement sensor in each case by a        line scan of the measurement beam by deflecting the processing        mirror scanner about the second tilt axis thereof, and        ascertaining an axis of the measurement coordinate system on the        basis of the captured points of intersection of the two line        scans with the height edge; and    -   determining a rotational deviation between the processing and        measurement coordinate systems on the basis of the ascertained        axes of the processing and measurement coordinate systems.

According to the invention, by means of a line scan of a linear heightedge provided on the workpiece support plane, that is to say of athree-dimensional surface feature, and the simultaneous deflection ofthe measurement and processing mirror scanners in each case about a tiltaxis, the rotational deviation of the two scanner coordinate systems isdetermined, which can then be compensated for by position regulation ofthe measurement scanner, for example.

The processing and measurement mirror scanners can each comprise abiaxial mirror tiltable about two tilt axes, or two uniaxial mirrorseach tiltable about one tilt axis.

Particularly preferably, in method step (a) the pinhole diaphragmdetector is arranged on the workpiece support plane in that position inwhich the processing beam impinges on the workpiece support plane as faras possible at right angles.

With further preference, in method step (a) the previously known imageposition lies in the image center of the sensor image of the measurementsensor. For this purpose, the previously known image position may havebeen adjusted exactly to the image center—e.g. during a precedingcalibration process.

Advantageously, in method step (b) the positive and negative fixedmagnitudes are in each case equal.

Very particular preferably, in method step (b) the height edge is anedge which has either a component arranged on the workpiece supportplane, or has been produced previously at a workpiece arranged on theworkpiece support plane by material removal by means of the processingbeam.

Preferably, the measurement beam reflected at the workpiece is deflectedbetween the measurement mirror scanner and a laser beam generator, inparticular coherence tomography apparatus, which emits the measurementbeam, in the direction of the measurement sensor.

A translational and/or rotational deviation between the processing andmeasurement coordinate systems that has been determined according to theinvention can be correspondingly corrected by position regulation of themeasurement scanner, e.g. by a machine controller.

The invention also relates to a computer program product comprising codemeans adapted for carrying out all the steps of the method describedabove when the program is executed on a controller of a laser processingmachine.

Further advantages and advantageous configurations of the subject matterof the invention are evident from the description, the claims and thedrawings.

Likewise, the features mentioned above and those referred to hereinaftercan be used in each case by themselves or as a plurality in any desiredcombinations.

The embodiments shown and described should not be understood as anexhaustive enumeration, but rather are of exemplary character foroutlining the invention. In the figures:

FIG. 1 schematically shows a laser processing machine suitable forcarrying out the method according to the invention for determiningtranslational and/or rotational deviations between the coordinatesystems of a processing mirror scanner and of a measurement mirrorscanner;

FIGS. 2a, 2b show a pinhole diaphragm detector arranged on a workpiecesupport plane for ascertaining an x-y focal position deviation of aprocessing beam in a perspective view (FIG. 2a ) and in a plan view(FIG. 2b );

FIG. 3 shows the sensor image of a spatially resolving measurementsensor for determining a translational deviation between the processingand measurement coordinate systems;

FIGS. 4a, 4b show a height edge arranged on a workpiece support planefor ascertaining the y-axis of a processing coordinate system (FIG. 4a )and for ascertaining the y-axis of a measurement coordinate system (FIG.4b ); and

FIG. 5 shows the sensor image of a spatially resolving measurementsensor for determining a rotational deviation between the processing andmeasurement coordinate systems.

The laser processing machine 1 shown in FIG. 1 serves for processingworkpieces 2 by means of a (laser) processing beam 3.

The laser processing machine 1 comprises a laser beam generator 4 forgenerating the processing beam 3, a first deflection mirror 5, whichdeflects the processing beam 3 by e.g. 90°, an optional seconddeflection mirror 6, which deflects the processing beam 3 once again bye.g. 90°, and a processing mirror scanner 7 for deflecting theprocessing beam 3 two-dimensionally in the direction of a workpiece 2arranged on a workpiece support plane 8. In the exemplary embodimentshown, the processing mirror scanner 7 is embodied as a mirror 9tiltable about two tilt axes A, B, i.e. a biaxial mirror, but canalternatively also be formed by two mirrors each tiltable about only onetilt axis A, B, i.e. uniaxial mirrors. The processing coordinate systemdefined by the two tilt axes A, B is designated by 10.

The laser processing machine 1 furthermore comprises a coherencetomography apparatus as measurement beam generator 11 for generating anOCT (laser) measurement beam 12, illustrated in a dashed manner, and ameasurement mirror scanner 13 for deflecting the measurement beam 12two-dimensionally onto the first deflection mirror 5, which istransmissive to the measurement beam 12 on both sides. In the exemplaryembodiment shown, the measurement mirror scanner 13 is embodied as amirror 14 tiltable about two tilt axes C, D, i.e. a biaxial mirror, butcan alternatively also be formed by two mirrors each tiltable about onlyone tilt axis C, D, i.e. uniaxial mirrors. The measurement coordinatesystem defined by the two tilt axes C, D is designated by 15. The tiltaxes A and C run parallel to one another, in the X-direction in theexemplary embodiment shown, and the tilt axes B and D run parallel toone another, in the Y-direction in the exemplary embodiment shown.

In FIG. 1, both the processing scanner 7 and the measurement scanner 13are shown in their so-called zero position. That is to say that the twoaxes A, B and C, D of the respective scanners 7, 13 each adopt a neutralreference position (zero position) shown in FIG. 1, in which positionthey do not effect targeted beam deflections. In the zero position ofthe measurement scanner 13, the measurement beam 12 is coupled into theprocessing beam 3 collinearly at the first deflection mirror 5. At theprocessing mirror scanner 7, both the processing beam 3 and themeasurement beam 12 are then deflected two-dimensionally in thedirection of the workpiece 2.

The laser processing machine 1 furthermore comprises a deflection mirror16 arranged between measurement beam generator 11 and measurement mirrorscanner 13, said deflection mirror being transmissive to the measurementbeam 12 coming from the measurement beam generator 11. The measurementbeam 12′ reflected at the workpiece 2 returns on the path of theincident measurement beam 12 and is deflected onto a spatially resolvingmeasurement sensor 17 by the deflection mirror 16, which is nontransmissive or partly transmissive in this direction. In the zeroposition of the measurement mirror scanner 13, the reflected measurementbeam 12′, in the sensor image 18 (FIG. 3) of the measurement sensor 17,is imaged onto a previously known image position 19, merely by way ofexample the image center in FIG. 3.

In order to determine a translational deviation between the processingand measurement coordinate systems 10, 15, the following procedure isimplemented:

As shown in FIGS. 2a, 2b , firstly an x-y focal position deviation ofthe processing beam 3 relative to the pinhole diaphragm center 20 of apinhole diaphragm detector 21 arranged on the workpiece support plane 8is ascertained according to the method known from DE 10 2011 006 553 A1.This is effected by scanning the pinhole diaphragm 22 with theprocessing beam 3 deflected by the processing mirror scanner 7 in an x-ygrid and by evaluating the laser power detected at each of the gridpoints 23 by a detector area 24 downstream of the pinhole diaphragm 20.The processing mirror scanner 7 is then fixed in the scan position whichhas been corrected on the basis of the ascertained x-y focal positiondeviation, in which scan position the focal position of the processingbeam 3 is situated exactly in the pinhole diaphragm center 18.

With the processing mirror scanner 7 fixed in this way, the height ofthe pinhole diaphragm 22 is captured in a spatially resolving manner bymeans of the measurement sensor 17 by scanning the pinhole diaphragm 22with the measurement beam 12 deflected by the measurement mirror scanner13.

As shown in FIG. 3, on the basis of the deviation—present in the sensorimage 18 of the measurement sensor 17—between the previously known imageposition 19 corresponding to the focal position of the processing beam 3and the pinhole diaphragm center 20′ of the pinhole diaphragm 22′captured in terms of height, a translational deviation Δx, Δy betweenthe processing and measurement coordinate systems 10, 15 can bedetermined.

Preferably, the pinhole diaphragm detector 21 is arranged on theworkpiece support plane 8 where the processing beam 3 impinges on theworkpiece support plane 8 as far as possible at right angles.

In order to determine a rotational deviation about the Z-axis betweenthe processing and measurement coordinate systems 10, 15, the followingprocedure is implemented:

As shown in FIG. 4a , firstly a component 25 having a linear height edge26 is placed on the workpiece support plane 8, specifically at 27, wherethe measurement beam 12 impinges on the workpiece support plane 8 in thezero positions of the processing and measurement mirror scanners 7, 13.Instead of being formed on a separate component 25, the height edge 26can also be formed on the pinhole diaphragm detector 21.

In a first step, as is furthermore shown in FIG. 4a , the measurementbeam 12 is deflected in each case by a positive and a negative fixedmagnitude +dy, −dy in the workpiece support plane 8 by tilting theprocessing mirror scanner 7 about the tilt axis A and, with theprocessing mirror scanner 7 fixed in each case in these tilted scanpositions, the height edge 26 is captured by means of the measurementsensor 17 in each case by a line scan 28 a, 28 b of the measurement beam12 by deflecting the measurement mirror scanner 13 about the tilt axisD. As shown in FIG. 5, the y_(BKS)-axis of the processing coordinatesystem 10 can then be ascertained in the sensor image 18 of themeasurement sensor 17 on the basis of the points of intersection 29 a,29 b—imaged there—of the two line scans 28 a, 28 b with the height edge26.

In a second step, as shown in FIG. 4b , the measurement beam 12 isdeflected in each case by a positive and a negative fixed magnitude +dy,−dy in the workpiece support plane 8 by tilting the measurement mirrorscanner 13 about the tilt axis C and, with the measurement mirrorscanner 13 fixed in each case in these tilted scan positions, the heightedge 26 is captured by means of the measurement sensor 17 in each caseby a line scan 30 a, 30 b of the measurement beam 12 by deflecting theprocessing mirror scanner 7 about the tilt axis B. As likewise shown inFIG. 5, the y_(MKS)-axis of the measurement coordinate system 15 canthen be ascertained in the sensor image 18 of the measurement sensor 17on the basis of the points of intersection 31 a, 31 b—imaged there—ofthe two line scans 30 a, 30 b with the height edge 26.

In a third step, as shown in FIG. 5, a rotational deviation Δα betweenthe processing and measurement coordinate systems 10, 15 is determinedon the basis of the angle of intersection of the ascertained axesy_(BKS), y_(MKS) of the processing and measurement coordinate systems10, 15.

The translational and rotational deviations Δx, Δy, Δa thus determinedcan be corrected for example by a machine controller of the laserprocessing machine 1 by displacement and rotation of the measurementmirror scanner 13.

Instead of the height edge 26 being provided on the component 25 or onthe pinhole diaphragm detector 21, the linear height edge 26 can also begenerated on a workpiece 2 situated on the workpiece support plane 8directly by means of a laser removal process, for example parallel tothe B, D axes.

The processing and measurement mirror scanners 7, 13 can also beembodied as 3D scanners, instead of as 2D scanners as described above,and so the respective laser focus can also be adjusted along theprocessing and measurement beams 3, 12, respectively, that is to say inthe Z-direction. For this purpose, a collimation lens 32 is arranged inthe beam path of the processing beam 3 between the laser beam generator4 and the processing mirror scanner 7, here merely by way of examplebetween the laser beam generator 4 and the first deflection mirror 5,said collimation lens being displaceable by means of a controlled axis33 along the processing beam 3. A collimation lens 34 is arranged in thebeam path of the measurement beam 12 between the measurement beamgenerator 11 and the measurement mirror scanner 13, here merely by wayof example between the deflection mirror 16 and the measurement mirrorscanner 13, said collimation lens being displaceable by means of acontrolled axis 35 along the measurement beam 12.

1-10. (canceled)
 11. A method for determining at least one oftranslational or rotational deviations between a measurement coordinatesystem of a measurement mirror scanner being tiltable about two axes anddeflecting a measurement beam two-dimensionally, and a processingcoordinate system of a processing mirror scanner being tiltable abouttwo axes and deflecting both the measurement beam deflected by themeasurement mirror scanner and a processing beam two-dimensionally ontoa workpiece, the measurement beam reflected at the workpiece returningon a path of the incident measurement beam and being captured by aspatially resolving measurement sensor to ascertain spatially resolvinginformation of the workpiece, and in a zero position of the measurementmirror scanner, the reflected measurement beam being imaged in a sensorimage of the measurement sensor, onto a previously known image position,the method comprising at least one of: step a) ascertaining an x-y focalposition deviation of the processing beam relative to a pinholediaphragm center of a pinhole diaphragm detector disposed on a workpiecesupport plane by scanning the pinhole diaphragm with the processing beamdeflected by the processing mirror scanner in an x-y grid and byevaluating a laser power detected at each of grid points, and by fixingthe processing mirror scanner in a scan position having been correctedbased on the ascertained x-y focal position deviation, the focalposition of the processing beam in the scan position being situated in apredetermined position; capturing spatially resolving height informationof the pinhole diaphragm, with the processing mirror scanner fixed in acorrected scan position, by using the measurement sensor to scan thepinhole diaphragm with the measurement beam deflected by the measurementmirror scanner; and determining a translational deviation between theprocessing and measurement coordinate systems based on a deviation,present in the sensor image of the measurement sensor, between thepreviously known image position corresponding to the focal position ofthe processing beam and the pinhole diaphragm center captured from theheight information; or step b) deflecting the measurement beam byrespective positive and negative fixed magnitudes in the workpiecesupport plane by tilting the processing mirror scanner about one, firsttilt axis thereof and, with the processing mirror scanner fixed in therespective tilted scan positions, capturing a linear height edgedisposed at the workpiece support plane by using the measurement sensorfor a respective line scan of the measurement beam by deflecting themeasurement mirror scanner about one, second tilt axis thereof, andascertaining an axis of the processing coordinate system based oncaptured points of intersection of the two line scans with the heightedge; deflecting the measurement beam by respective positive andnegative fixed magnitudes in the workpiece support plane by tilting themeasurement mirror scanner about the other, first tilt axis thereof and,with the measurement mirror scanner fixed in the respective tilted scanpositions, capturing the height edge by using the measurement sensor forrespective line scans of the measurement beam by deflecting theprocessing mirror scanner about the second tilt axis thereof, andascertaining an axis of the measurement coordinate system based on thecaptured points of intersection of the two line scans with the heightedge; and determining a rotational deviation between the processing andmeasurement coordinate systems based on the ascertained axes of theprocessing and measurement coordinate systems.
 12. The method accordingto claim 11, which further comprises providing the predeterminedposition as the pinhole diaphragm center of the pinhole diaphragmdetector.
 13. The method according to claim 11, which further comprisesproviding each of the processing mirror scanner and the measurementmirror scanner with one respective mirror tiltable about two tilt axesor two mirrors each tiltable about one tilt axis.
 14. The methodaccording to claim 11, which further comprises in step placing thepinhole diaphragm detector on the workpiece support plane where theprocessing beam impinges on the workpiece support plane at right angles.15. The method according to claim 11, which further comprises in steplocating the previously known image position in an image center of thesensor image.
 16. The method according to claim 11, which furthercomprises in step setting the positive and negative fixed magnitudes tobe equal.
 17. The method according to claim 11, which further comprisesbefore step placing a component having the height edge on the workpiecesupport plane.
 18. The method according to claim 11, which furthercomprises before step producing the height edge at a workpiece disposedon the workpiece support plane by material removal provided by theprocessing beam.
 19. The method according to claim 11, which furthercomprises in step deflecting the measurement beam, reflected at theworkpiece, between the measurement mirror scanner and a laser beamgenerator emitting the measurement beam in a direction of themeasurement sensor.
 20. The method according to claim 19, which furthercomprises providing a coherence tomography apparatus as the laser beamgenerator.
 21. The method according to claim 11, which further comprisescompensating for at least one of the determined translational orrotational deviation by position regulation of at least one of theprocessing mirror scanner or the measurement mirror scanner.
 22. Anon-transitory computer-readable medium with instructions storedthereon, that perform the steps of claim 11 when executed on a processorof a laser processing machine.